For example, games like Battletoads or Ninja Gaiden might be difficult, but you will only lose due to lack of skill. On the other hand, Touhou or a battle with an enemy with SNK boss syndrome are made difficult by unfair means and have a ton of cheap death.

Call of Duty on veteran mode is full of cheap deaths because the enemies infinitely respawn until you get to a certain point, they all have pinpoint aim, and they all have infinite grenades.